Mean Hadley Circulation Research Articles

Wettening of the Southern Hemisphere Land Monsoon during 1901–2014

Abstract The causes of historical changes in the Southern Hemisphere (SH) monsoon are less understood than the Northern Hemisphere (NH) counterpart. Unlike the decline in the NH monsoon during 1901–2014, we found that the SH land monsoon precipitation significantly increased during 1901–2014 in observation, reanalysis, and most historical simulations from phase 6 of the Coupled Model Intercomparison Project (CMIP6). The observed increase in SH land monsoon precipitation is dominated by the Australian and South American monsoons. Moisture budget analysis suggests that half of the wettening is attributable to the strengthening of monsoon circulation, and only one-fifth is caused by atmospheric moistening. The SH monsoon circulation change is mainly affected by the sea surface temperature (SST) gradient between the Indo-Pacific and the eastern Pacific. It enhances the tropical zonal circulation that redistributes the moisture from tropical oceans to land monsoon regions by strengthening the lower-tropospheric convergence and convection. The CMIP6 models, which successfully reproduced the SST contrast between the Indo-Pacific and eastern Pacific, simulate the wettening of the SH monsoon during the historical period; otherwise, the SH monsoon is weakened. In a meridional sense, reanalysis and CMIP6 simulations both demonstrated that the strengthening of SH monsoon convection plays a vital role in the long-term change of zonal mean Hadley circulation, albeit the monsoon band only accounts for 1/3 of the global longitudinal area. Results from this study are useful for constraining the future projection of SH monsoon and understanding the long-term change of Hadley circulation.

Inter-model spreads of the climatological mean Hadley circulation in AMIP/CMIP6 simulations

Inter-model spreads of the climatological mean Hadley circulation in AMIP/CMIP6 simulations

A rare episode of minor circulation embedded in the northern hemispheric zonal mean hadley cell

A rare episode of minor circulation embedded in the Hadley circulation (HC) and processes responsible for its formation is discussed. Using 34 years (1979–2012) of HC climatology derived from reanalysis datasets, a minor circulation centered on ∼35°N and embedded within the northern hemispheric zonal mean HC is observed during the month of July 1993. A longitudinally resolved vertical velocity observations centered on 35°N latitudinal belt revealed that there is an anomalous upwelling over the North American sector possibly associated with “Great Floods of 1993” thus emphasizing the prominence of a regional feature and its impact on the zonally averaged circulation.

Tropical moist convection as an important driver of Atlantic Hadley circulation variability

AbstractThe exact role of moist deep convection and associated latent heating in the tropical Hadley circulation has been debated for many years. This study investigates the connection between moist convection and the strength of the upper‐level meridional circulation over the tropical Atlantic, focusing mainly on one particular boreal winter season. There is a close relationship between events of strong organised deep convection and enhanced meridional upper‐level wind on many occasions. A process‐based analysis of specific events suggests that moist convection impacts Hadley circulation variability on time‐scales of days to months through equatorial wave dynamics. Equatorial waves play an important role, both directly by contributing to the Hadley circulation via their meridional wind component and also indirectly by triggering moist convection through low‐level convergence. Specific Hadley circulation surge events, short‐term, regionally confined intensifications of the upper‐level meridional circulation, can be attributed to enhanced organised moist convection and equatorial wave activity in many cases, with implications for trade wind cloudiness. The findings thus elucidate how the mean Hadley circulation is shaped by and composed of temporally and spatially varying convection–circulation interactions.

Inter-model spread of the climatological annual mean Hadley circulation and its relationship with the double ITCZ bias in CMIP5

In the present paper, we study the climatological annual mean Hadley circulation simulated by 47 coupled atmospheric and ocean general circulation models (AOGCMs) of the phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our results show that the climatological annual mean states of the Hadley circulation, i.e., strength, central line, and poleward boundaries of the Hadley cells, have a large inter-model spread. The strength of the Hadley cells, i.e., the maximum value of mean meridional streamfunction (MMS), varies from 7.0 × 1010 to 11.1 × 1010 kg s− 1. The variation range is about half of the multi-model ensemble mean strength of about 8.7 × 1010 kg s− 1. The central line location between the two Hadley cells varies from 3.7° S to 7.8° N, and the poleward boundary latitudes have a variation range of 5.5° in latitude. In contrast, inter-model spreads of the Hadley circulation are much smaller in reanalyses. Using a method of inter-model empirical orthogonal function (EOF) analysis of annual-mean MMS, we show that the first principal component (PC1) is closely correlated with the central line latitudes of the Hadley cells, and that the second principal component (PC2) has a high correlation with the poleward boundaries. Both PC1 and PC2 are significantly correlated with the strength of the Hadley cells. Regressions of sea surface temperatures (SSTs) and precipitation on PC1 and PC2 show that EOF1 is associated with the well-known double intertropical convergence zone (ITCZ) problem in AOGCMs, and that EOF2 is related to the cold tongue bias. The results suggest that the large inter-model spread of the climatological mean Hadley circulation is largely due to the double ITCZ and cold tongue biases, and that the key for improving the simulation performance of the Hadley circulation is to reduce the double ITCZ and cold tongue bias in AOGCMs.

On the discrepancies in the changes in the annual mean Hadley circulation among different regions and between CMIP5 models and reanalyses

Recent studies have investigated past and future changes in the intensity and sinking branch of the global zonally averaged Hadley circulation (ZAHC), and discrepancies were found between climate models and reanalyses. However, studies concerning the regional Hadley circulation (HC), especially based on model simulations, are relatively rare. Here, using reanalyses and CMIP5 simulation datasets, we study the long-term changes in regional HCs over Africa (AFHC), the Indian Ocean (IOHC), the western Pacific (WPHC), the eastern Pacific (EPHC), South America (SAHC), and the Atlantic (ATHC). The results obtained from the reanalyses show that during the historical period, the intensification of the WPHC, EPHC, SAHC, and ATHC in the Northern Hemisphere (NH) may contribute to the strengthening of the ZAHC in the NH, while the contributions of the NH AFHC and IOHC are relatively small. The intensification of the IOHC, EPHC, SAHC, and ATHC in the Southern Hemisphere (SH) may contribute to the strengthening of the ZAHC in the SH, while the contributions of the SH AFHC and WPHC are negative. Regarding the poleward edge, the NH HCs have generally shifted poleward in recent decades, while trends in the SH HCs are nonsignificantly greater than zero in most cases. As reanalysis trends in the HC fall within the range of the model trends, we do not find sufficient evidence to demonstrate a discrepancy in the long-term changes in the HC between the CMIP5 models and reanalyses. The correlations of the HC intensities (poleward edges) among different regions obtained from the reanalysis data are generally different than (similar to) those obtained from the CMIP5 historical simulations, and the difference (similarity) may be caused by the difference (similarity) in the internal variabilities between the reanalyses and CMIP5 preindustrial control simulations. The changes in these HCs under global warming conditions are also investigated in this study.

Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends

AbstractThe width of the tropical Hadley circulation (HC) has garnered intense interest in recent decades, owing to the emerging evidence for its expansion in observations and models and to the anticipated impacts on surface climate in its descending branches. To better clarify the causes and impacts of tropical widening, this work generalizes the zonal mean HC to the regional level by defining meridional overturning cells (RC) using the horizontally divergent wind. The edges of the RC are more closely connected to surface hydroclimate than more traditional metrics of regional tropical width (such as the sea level pressure ridge) or even than the zonal mean HC. Simulations reveal a robust weakening of the RC in response to greenhouse gas increases, along with a widening of the RC in some regions. For example, simulated widening of the zonal mean HC in the Southern Hemisphere appears to arise in large part from regional overturning anomalies over the Eastern Pacific, where there is no clear RC. Unforced interannual variability in the position of the zonal mean HC edge is associated with a more general regional widening. These distinct regional signatures suggest that the RCs may be well suited for the attribution of observed circulation trends. The spatial pattern of regional meridional overturning trends in reanalyses corresponds more closely to the pattern associated with unforced interannual variability than to the pattern associated with CO2 forcing, suggesting a large contribution of natural variability to the recent observed tropical widening trends.

Contrast in monsoon precipitation over oceanic region of north Bay of Bengal and east equatorial Indian Ocean

ABSTRACTThis study explores the possible causes of rainfall distribution over the two major oceanic raining regions of the north Bay of Bengal (nBoB) and the east equatorial Indian Ocean (eEIO). Despite 17% difference in vertically averaged humidity, there is almost 34% difference in mean rainfall over these two regions. The climatological seasonal [June–September (JJAS)] mean (standard deviation) rainfall over nBoB region is always higher (lower) than that over the eEIO region in all the independent data used. The eEIO region has a much larger percentage of low stratiform and convective rainfall (<5 mm day−1) distribution as compared to nBoB, which is totally opposite in case of moderate stratiform and convective rainfall (>5 mm day−1) distribution. This is further substantiated by a much lower values of outgoing long‐wave radiation (OLR) in nBoB (<200 W m−2) as compared to the eEIO (217 W m−2) region. Mean Hadley circulation along with relative vorticity/divergence profile supports more intense (gentle) updrafts over nBoB (eEIO) region. Latent heat (LH) is almost three times at the upper level (∼8 km) in case of nBoB as compared to eEIO; however, at the lower level (∼3 km) LH is marginally higher over eEIO region. Microphysical variables, namely cloud ice optical thickness and cloud ice water path, are in much larger quantities over nBoB as compared to eEIO. Furthermore, the cold (warm) rain processes dominate among other microphysical processes over nBoB (eEIO) region. Thus, the interplay among large‐scale dynamics, thermodynamics and microphysics is very crucial in the formation of deep clouds and convective rain over the nBoB region and similarly shallow clouds and stratiform rain over the eEIO region. This study will be very useful to guide present‐day coupled models for proper representation of different rain components over the nBoB and eEIO region.

Impact of Mountains on Tropical Circulation in Two Earth System Models

Abstract Two state-of-the-art Earth system models (ESMs) were used in an idealized experiment to explore the role of mountains in shaping Earth’s climate system. Similar to previous studies, removing mountains from both ESMs results in the winds becoming more zonal and weaker Indian and Asian monsoon circulations. However, there are also broad changes to the Walker circulation and El Niño–Southern Oscillation (ENSO). Without orography, convection moves across the entire equatorial Indo-Pacific basin on interannual time scales. ENSO has a stronger amplitude, lower frequency, and increased regularity. A wider equatorial wind zone and changes to equatorial wind stress curl result in a colder cold tongue and a steeper equatorial thermocline across the Pacific basin during La Niña years. Anomalies associated with ENSO warm events are larger without mountains and have greater impact on the mean tropical climate than when mountains are present. Without mountains, the centennial-mean Pacific Walker circulation weakens in both models by approximately 45%, but the strength of the mean Hadley circulation changes by less than 2%. Changes in the Walker circulation in these experiments can be explained by the large spatial excursions of atmospheric deep convection on interannual time scales. These results suggest that mountains are an important control on the large-scale tropical circulation, impacting ENSO dynamics and the Walker circulation, but have little impact on the strength of the Hadley circulation.

Onset and withdrawal of the large‐scale South Asian monsoon: A dynamical definition using change point detection

AbstractWe introduce an objective definition for onset and withdrawal of the South Asian summer monsoon (SASM), based on the large‐scale atmospheric moisture budget. The change point (CHP) index allows precise characterization of the different stages and timescales of the large‐scale SASM and is highly correlated with the local operational index, the monsoon onset over Kerala. The CHP‐based onset and withdrawal dates, which capture the expected seasonal transitions in rainfall and winds, correspond with regime changes in the SASM moisture budget between negative and positive net precipitation. Climatological composites reveal that the seasonal transitions in SASM sector mean precipitation and circulation closely resemble those of the zonal mean Hadley circulation. The CHP index at grid points within the SASM domain yields a robust definition of local onset and withdrawal dates, which are well correlated with the large‐scale index on interannual timescales, providing insight into the regional variability of the SASM.

The Multidecadal Variability of the Asymmetric Mode of the Boreal Autumn Hadley Circulation and Its Link to the Atlantic Multidecadal Oscillation

Abstract Previous studies show that the first principal mode of the variability of the seasonal mean Hadley circulation (HC) is an equatorial asymmetric mode (AM) with long-term trend. This study demonstrates that the variability of the boreal autumn [September–November (SON)] HC is also dominated by an AM, but with multidecadal variability. The SON AM has ascending and descending branches located at approximately 20°N and 20°S, respectively, and explains about 40% of the total variance. Further analysis reveals that the AM is closely linked to the Atlantic multidecadal oscillation (AMO), which is associated with a large cross-equatorial sea surface temperature (SST) gradient and sea level pressure (SLP) gradient. The cross-equatorial thermal contrast further induces an equatorial asymmetric HC anomaly. Numerical simulations conducted on an atmospheric general circulation model also suggest that AMO-associated SST anomalies can also induce a cross-equatorial SLP gradient and anomalous vertical shear of the meridional wind at the equator, both of which indicate asymmetric HC anomaly. Therefore, the AM of the variability of the boreal autumn HC has close links to the AMO. Further analysis demonstrates that the AMO in SON has a closer relationship with AM than those in the other seasons. A possible reason is that the AMO-associated zonal mean SST anomaly in the tropics has differences among the four seasons, which leads to different atmospheric circulation responses. The AM in SON has inversed impacts on the tropical precipitation, suggesting that the precipitation difference between the northern and southern tropics has multidecadal variability.

Climatology and interannual variability of the annual mean Hadley circulation in CMIP5 models

Using 26 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), climatology and the interannual variability of the annual mean Hadley circulation are evaluated. The results show that most of 26 models perform well in simulating the spatial structure of the climatology of the annual mean Hadley circulation, but the results derived from these models are generally weaker than that derived from the reanalysis dataset. Eighteen models can properly simulate well the asymmetric mode and symmetric mode of the annual mean Hadley circulation variability. Two models can only simulate asymmetric mode or symmetric mode and the other two models simulate reversed sequences of asymmetric mode and symmetric mode.The possible reason why some models cannot properly simulate the asymmetric mode and symmetric mode is that these models do not properly simulate the structure of zonal mean sea surface temperature (SST). Especially, not properly simulating variances of symmetric and asymmetric components of the SSTA will lead to reversed sequence of symmetric mode and asymmetric mode. And not properly simulated either symmetric or asymmetric component of the SSTA will lead to inability in simulating symmetric mode or asymmetric mode. On the other hand, some models properly simulate the asymmetric mode and symmetric mode, but do not properly simulate the responses to SST change. These models can not reflect the air sea coupling processes in associated with the Hadley circulation, therefore they should be taken more care when classify the models into groups.

Interannual variability of the Hadley circulation associated with tropical Pacific SST anomaly

The seasonal and interannual variability of zonal mean Hadley circulation are analyzed, and the important effects of sea surface temperature (SST), especially the tropical Pacific SST, on the meridional circulation are discussed. Following results are obtained: 1) the Hadley circulation presents a single clockwise (anticlockwise) cross-equator circulation in the Northern (Southern) Hemisphere winter,while it is a double-ring-shaped circulation quasi-symmetric about the equator in spring and autumn. The annual mean state just indicates the residual of the Hadley cell in winter and summer. 2) The first mode of interannual anomalies shows a single cell crossing the equator like the climatology in winter and summer but with narrower width. The second mode shows a double ring-shaped cell quasi-symmetric about the equator which is similar to the Hadley cell in spring or autumn. 3) Vertical motion of the Hadley circulation is driven by sea surface temperature (SST) through latent and sensible heat in the tropics, and the interannual anomalies are mainly driven by the SST anomaly (SSTa) in the tropical Pacific. 4) The meridional gradient of SSTa is well consistent with the lower meridional wind of Hadley circulation in the interannual part. For the spatial distribution, the meridional gradient of SSTa in the Pacific plays a major role for the first two modes while the effects of the Indian Ocean and the Atlantic Ocean can be ignored.

Simulation of the equatorially asymmetric mode of the Hadley circulation in CMIP5 models

The tropical Hadley circulation (HC) plays an important role in influencing the climate in the tropics and extra-tropics. The realism of the climatological characteristics, spatial structure, and temporal evolution of the long-term variation of the principal mode of the annual mean HC (i.e., the equatorially asymmetric mode, EAM) was examined in model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The results showed that all the models are moderately successful in capturing the HC’s climatological features, including the spatial pattern, meridional extent, and intensity, but not the spatial or temporal variation of the EAM. The possible reasons for the poor simulation of the long-term variability of the EAM were explored. None of the models can successfully capture the differences in the warming rate between the tropical Southern Hemisphere (SH) and Northern Hemisphere (NH), which is considered to be an important driver for the variation of the AM. Most of the models produce a faster warming in the NH than in the SH, which is the reverse of the observed trend. This leads to a reversed trend in the meridional gradient between the SH and NH, and contributes to the poor simulation of EAM variability. Thus, this aspect of the models should be improved to provide better simulations of the variability of the HC. This study suggests a possible reason for the poor simulation of the HC, which may be helpful for improving the skill of the CMIP5 models in the future.

Regional changes in the annual mean Hadley circulation in recent decades

Previous studies suggest that the zonally averaged Hadley circulation (ZAHC) has experienced a robust poleward expansion, and its trend in intensity displays inconsistency among different data sets. This study examines changes in regional HC intensity and poleward edge using six reanalyses, outgoing longwave radiation, and precipitation data sets. HCs in six regions, including Africa (AFHC), the Indian Ocean (IOHC), the western Pacific (WPHC), the eastern Pacific (EPHC), South America (SAHC), and the Atlantic (ATHC), are investigated. Intensity trends in the Northern Hemisphere (NH) WPHC and ATHC and the Southern Hemisphere (SH) EPHC and ATHC are in agreement with each other in the six reanalyses. Furthermore, regional HCs in these domains appear to be intensifying, although not all of the reanalyses show statistically significant trends. For the poleward edge, its trend in the NH AFHC, IOHC, EPHC, SAHC, and ATHC is significantly larger than zero, and the northern HC poleward edge exhibits uniform poleward migrations in these five regions. In the SH, only the trend in the SAHC poleward edge is significantly different from zero. Furthermore, the trend in the SH SAHC poleward edge is significantly larger than those in the SH AFHC, IOHC, and ATHC. The results indicate that the poleward migration of the southern ZAHC poleward edge during recent decades that has been identified by previous studies may be attributed mainly to the poleward migration of the southern SAHC poleward edge. Further analyses suggest that changes in regional HC poleward edges could have a significant impact on regional precipitation anomalies.

A comparison of the Hadley circulation in modern reanalyses

[1] Previous studies using reanalysis data suggest an intensification and poleward expansion of the tropical Hadley circulation (HC) throughout the twentieth century, yet the HC climatology and trends remain undocumented for many of the newest reanalyses. An intercomparison of eight reanalyses is presented to better elucidate the mean state variability and trends concerning HC intensity and width. Significant variability between reanalyses was found in the mean HC intensity with less variability in HC width. Certain reanalyses (e.g., ERA40 and the Climate Forecast System Reanalysis) tend to produce stronger meridional overturning, while others (National Centers for Environmental Prediction–National Center for Atmospheric Research and Modern-Era Retrospective-Analysis for Research and Applications) are constantly weaker. The NOAA–Cooperative Institute for Research in Environmental Sciences Twentieth Century Reanalysis best matched the ensemble averages with the exception of a poleward shift in the subtropical terminus. Ensemble trends regarding HC intensity and width are broadly consistent with previous work, indicating a 0.40 (0.07) × 1010 kg s−1 decade−1 intensification in the northern (southern) cell and a 1.1° decade−1 widening in the past 30 years, although some uncertainty remains regarding the intensity of the southern cell. Longer-term ensemble trends (i.e., 1958–2008) containing fewer ensemble members suggest a weaker northern cell intensification but stronger southern cell intensification and a more modest widening of the HC (i.e., 0.53° decade−1) compared to the last 30 years. Separation of the seasonally averaged stream function magnitudes by the El Nino–Southern Oscillation (ENSO) phase revealed a weak clustering and statistically significant strengthening of the mean circulation for El Nino compared to ENSO neutral and La Nina events for the winter cell with little difference in the summer cell intensity.

Modeled sensitivity of upper thermocline properties to tropical cyclone winds and possible feedbacks on the Hadley circulation

The sensitivity of upper thermocline properties, and global climate, to tropical cyclone (TC) winds is examined using global ocean and atmosphere general circulation models. We combine seven years of global, satellite‐based TC wind records with a standard surface wind input data set derived from reanalysis, and we apply idealized factors to TC winds in order to model the ocean's equilibrium response to increases in TC intensities. We find TC‐induced vertical ocean mixing impacts upper thermocline properties, such as temperature and mixed layer depth, and the effects are amplified for increasing intensities. The model's ocean heat transport is also affected, but only when TC winds are increased substantially compared to present‐day values. Atmospheric model simulations show altered ocean temperature can lead to changes in the mean Hadley circulation. Results suggest increased TC activity may affect global climate by altering the ocean's thermal structure, which could be important for large scale ocean‐atmosphere feedbacks.

Formation mechanisms of latitudinal CO2 gradients in the upper troposphere over the subtropics and tropics

Aircraft observations and numerical simulations using an atmospheric transport model exhibit large latitudinal gradients in the carbon dioxide (CO2) mixing ratio around the subtropics and equator throughout the troposphere. The formation mechanisms of the latitudinal CO2 gradients are investigated at aircraft flight altitudes (350–260 hPa) by considering the influences of atmospheric transport and carbon fluxes at the earth's surface. A meridional transport analysis demonstrates how various transport processes create the latitudinal CO2 gradients in the upper troposphere. The analysis result shows that around the northern subtropics, the suppression of meridional mixing sharpens the CO2 gradient during boreal winter and spring. In other words, extratropical cyclonic activity effectively flattens the mixing ratio along isentropic surfaces at midlatitudes and induces gradients around the subtropics. The southern subtropical CO2 gradient is also induced by the subtropical mixing barrier and convergence of the cross‐equatorial eddy flux. Different from the subtropical gradients, a CO2 gradient in the tropical upper troposphere is not created by the suppression of meridional transport but is created by the uplifting of low‐level air during boreal winter and spring. The latitudinal CO2 gradient in the tropical upper troposphere decreases due to interhemispheric transport. The seasonal migration of the mean Hadley circulation yields efficient interhemispheric transport and reduces the tropical CO2 gradient.

Hemispherical Asymmetry of Tropical Precipitation in ECHAM5/MPI-OM during El Niño and under Global Warming

Abstract Similarities and differences between El Niño and global warming are examined in hemispherical and zonal tropical precipitation changes of the ECHAM5/Max Planck Institute Ocean Model (MPI-OM) simulations. Similarities include hemispherical asymmetry of tropical precipitation changes. This precipitation asymmetry varies with season. In the boreal summer and autumn (winter and spring), positive precipitation anomalies are found over the Northern (Southern) Hemisphere and negative precipitation anomalies are found over the Southern (Northern) Hemisphere. This precipitation asymmetry in both the El Niño and global warming cases is associated with the seasonal migration of the Hadley circulation; however, their causes are different. In El Niño, a meridional moisture gradient between convective and subsidence regions is the fundamental basis for inducing the asymmetry. Over the ascending branch of the Hadley circulation, convection is enhanced by less effective static stability. Over the margins of the ascending branch, convection is suppressed by the import of dry air from the descending branch. In global warming, low-level moisture is enhanced significantly due to warmer tropospheric temperatures. This enhances vertical moisture transport over the ascending branch of the Hadley circulation, so convection is strengthened. Over the descending branch, the mean Hadley circulation tends to transport relatively drier air downward, so convection is reduced.

Asymmetric Responses of Tropical Precipitation during ENSO

Abstract In response to the zonally symmetric El Niño–Southern Oscillation forcing, hemispherically asymmetric tropical precipitation anomalies associated with the Hadley circulation are found. In boreal spring after an El Niño peak phase, positive tropical precipitation anomalies occur in the Southern Hemisphere, while negative precipitation anomalies are found in the Northern Hemisphere. This zonal asymmetry is more apparent in the El Niño decaying phase than in the El Niño growing phase. The maximum amplitude of this zonal asymmetry lags one season behind the maximum SST anomalies over the tropical eastern Pacific. This lagged response of the asymmetry is mainly because of the tropical precipitation outside the tropical eastern Pacific, which is associated with the SST and tropospheric temperature anomalies outside the tropical eastern Pacific. A combination of the effect associated with the anomalous gross moist stability and the effect of the horizontal moist static energy (MSE) transport is responsible for the asymmetry. The above effects are associated with the seasonal migration of the Hadley circulation. Warm SST and tropospheric temperature anomalies increase the low-level moisture in the Tropics. In the effect associated with anomalous gross moist stability, the tropical precipitation over the ascending branch of the Hadley circulation is enhanced because of the decrease of effective moist stability, which is induced by the increase of low-level moisture. This enhancement associated with the Hadley circulation reduces the low-level moisture over the descending branch and creates a meridional moisture gradient. In the effect of the horizontal MSE transport, the tropical precipitation anomalies over margins of the ascending branch is reduced by dry advection from the descending branch, which is associated with mean Hadley circulation.